The invention relates to a spring steel wire rod, and a method for producing the same, and more particularly, to a spring steel wire rod excellent in decarburization resistance, and satisfactory in wire drawing workability without undergoing ferritic decarburization otherwise occurring in a hot-rolling process, and a quenching process, and a method for producing the same.
A spring steel wire rod for use in production of a suspension spring, and so forth is normally produced by the steps of heating a billet, hot rolling the billet to be reduced to a wire rod of a predetermined wire diameter, subsequently winding the same in the form of a wound wire coil to be then cooled. The spring steel wire rod produced as above is thereafter subjected to processes of wire drawing→quenching and tempering→setting→shot peening to be subsequently turned into a spring.
Characteristics required of the spring include inhibition of decarburization (ferritic decarburization). The ferritic decarburization is a phenomenon accompanying transformation of austenite to ferrite, and occurs due to surface decarburization of a spring steel wire rod in the process of tempering besides the hot rolling process. Since the inhibition of the ferritic decarburization brings about various advantages such as omission of a peeling process for cutting away a decarburized layer, and so forth, yield enhancement, and so forth, besides ensuring of spring fatigue characteristics, there have since been made various proposals for inhibiting the ferritic decarburization. Technologies for inhibiting occurrence of a ferritic decarburized layer have been proposed by, for example, controlling composition of steel as disclosed in Patent Documents 1 and 2, or controlling heating temperature at the time of hot rolling, a cooling rate after the rolling, and so forth as disclosed in Patent Documents 3, and 4.
Further, excellence in wire drawing workability is required of a spring. For the spring steel wire rod, use is normally made of steel containing carbon in amounts in a range of about 0.35 to 0.65% from the viewpoint of ensuring strength, and so forth, so that there often occurs an increase in hardness after hot rolling, resulting in occurrence of a break in wire, and a crack at the time of wire drawing applied later on. Accordingly, various technologies for implementing enhancement of wire drawing workability of the spring steel wire rod have been proposed, and for example, in Patent Document 5, there has been described a method for improving the characteristics described as above by controlling composition of steel.
However, there has not been developed as yet a technology whereby the inhibition of the ferritic decarburization is rendered compatible with the enhancement of the wire drawing workability.    Patent Document 1: JP-A No. 2004-10965    Patent Document 2: JP-A No. 2003-105496    Patent Document 3: JP-A No. 2003-268433    Patent Document 4: JP-A No. 2002-194432    Patent Document 5: JP-A No. 2003-253391